(1) Field of the Invention
The present invention relates to an improvement in an electrostatic photographic copying process using a one-component type magnetic developer. More particularly, the present invention relates to an improvement for increasing the density and sharpness of a transferred image in the copying process comprising developing a latent image with a one-component type magnetic developer and transferring the resulting image of the developer to a transfer sheet from a substrate to obtain a copied image on the transfer sheet.
(2) Description of the Prior Art
As a developer capable of developing an electrostatic latent image without use of a particular carrier, there has been broadly known a so-called one-component type magnetic developer comprising a finely divided magnetic material incorporated in particles of a developer.
As one type of such one-component magnetic developer, there is known a so-called conductive magnetic developer formed by incorporating a finely divided magnetic material into developer particles to impart a property of being magnetically attracted to the developer particles and distributing a conducting agent such as electrically conductive carbon black on the surfaces of the developer particles (see, for example, the specifications of U.S. Pat. Nos. 3,639,245 and 3,965,022). When this conductive magnetic developer is caused to fall in the form of a magnetic brush in contact with an electrostatic latent image-carrying substrate to effect development of the electrostatic latent image, an excellent visible image free of so-called edge effect or fog is obtained. However, it is known that serious problems are caused when the image of this developer is transferred from the substrate to an ordinary transfer sheet. More specifically, as disclosed in Japanese patent application Laid-Open Specification No. 117435/75, when the specific resistance of a transfer sheet used is lower than 3.times.10.sup.13 .OMEGA.-cm as in case of ordinary plain paper, broadening of contours or reduction of the transfer efficiency is caused by scattering of the developer particles at the transfer step. This defect can be moderated to some extent by coating a highly electrically resistant resin, wax or oil on the toner-receiving face of a transfer sheet, but this improving effect is relatively low under high humidity conditions. Furthermore, the cost of transfer sheets is increased by coating of the above-mentioned resin, wax or oil and another defect of reduction of the touch is caused.
As another type of the one-component magnetic developer, there is known a non-conductive one-component magnetic developer comprising particles of a homogeneous mixture of a finely divided magnetic material and an electricity-detecting binder. For example, the specification of U.S. Pat. No. 3,645,770 discloses an electrostatic photographic copying process comprises charging a magnetic brush (layer) of the above-mentioned non-conductive magnetic developer with a polarity reverse to that of an electrostatic latent image to be developed by corona discharge, causing the charged developer to fall in contact with an electrostatic latent image-carrying substrate to develop the latent image and transferring the formed image of the developer to a transfer sheet. This process is advantageous in that a transfer image can be formed on so-called plain paper. However, it is difficult to uniformly charge the magnetic brush of the non-conductive magnetic developer to the deep root thereof and therefore, it is difficult to form an image having a sufficiently high density. Furthermore, since a corona discharge mechanism has to be disposed in a developing zone, this process involves a defect that the structure of the copying apparatus as a whole becomes complicated.
Recently, there have been proposed a process in which development of an electrostatic latent image is performed by utilizing charging of the developer by friction between a non-conductive magnetic developer and the surface of an electrostatic latent image-carrying substrate (see Japanese patent application Laid-Open Specification No. 62638/75) and a process in which development is performed by utilizing dielectric polarization of a non-conductive magnetic developer (see Japanese patent application Laid-Open Specification No. 133026/76).
In the former process, it is necessary to control developing conditions strictly, and if the development conditions are not strictly controlled, fogging is caused in a non-image area (especially conspicuous when the degree of the mutual contact between the surface of the photosensitive material and the tops of spikes of magnetic toner particles is high), and fixation of magnetic toner particles to a developing sleeve and blocking of magnetic toner particles are readily caused and this trouble is especially conspicuous when reproduction is carried out continuously.
In the latter process, the problem of fogging is not caused, but since a visible image is formed by applying to an electrostatic latent image a developing charge by the dielectric polarizing effect induced by the magnetic toner, a low voltage area of the electrostatic latent image is not advantageously developed. Accordingly, a low density portion of the original is not effectively reproduced and formation of a print of a half-tone image is difficult.
Furthermore, both of these two processes are defective in that obtained prints are inferior in the image sharpness, and when a p-type photosensitive material such as selenium is used for a photosensitive plate, images having a high density can hardly be formed according to these processes.